Mobile devices such as smartphones or tablets are equipped with an ever increasing number of sensors. Optical sensors are used to measure and detect ambient light, colour, gestures and proximity of objects, for example. Data collected by the sensors can be used by applications running on the mobile device and offer a vast amount of functionality. At the same time mobile devices typically rely on batteries as a power supply which only have limited capacity.
Not all components of a mobile device or the device itself need to be active at all times. In fact, there is an increasing demand in the market for a long range proximity sensor in order to wake up electronic devices such as computer monitors, laptops, printers, soda dispensers, and many others. One general motivation for “waking up” an electronic device from a “sleep” mode, is to save power. Another application is presence detection in a room to automatically turn off or on the device and/or lights in the absence or presence of motion.
Active proximity or gesture detection usually requires infrared light emitting diodes (IR LEDs) to emit pulses of light towards a potential target. Periodic polling must be done at all times to acquire a new proximity or gesture event. It is clear that this draws considerable power from the mobile device's batteries. Furthermore, active optical proximity and gesture sensors often have limited range. The typical useful range is only about 100 mm to 150 mm.
Passive proximity motion or presence detection offers much greater detection distance than existing optical or infrared proximity detection, while consuming considerably less power. If passive motion detection is used, motion events can be used to wake up the proximity and/or gesture sensors in the mobile devices so they only draw power when needed. This can save considerable power as there is no emission of light and no need for periodic polling and motion could be sensed at a considerably longer range while using little power.
Proximity infrared (PIR) sensors typically are used for passive motion and presence detection. At the core of a PIR sensor is a solid state sensor or set of sensors, made from pyroelectric materials, i.e. materials which generate energy when exposed to heat. Such sensors generate a temporary voltage when they are heated or cooled. However, PIR sensors typically cost considerably more than optical devices and are not nearly as compact.